Dual-stage, separated gas/fluid shock strut servicing monitoring system using two pressure/temperature sensors

ABSTRACT

A monitoring system for a dual-stage, separated gas/fluid shock strut may comprise a controller and a tangible, non-transitory memory configured to communicate with the controller. The tangible, non-transitory memory may have instructions stored thereon that, in response to execution by the controller, cause the controller to perform various operations. Said operations may include determining, by the controller, a shock strut stroke at which a secondary chamber of the shock strut is activated, calculating, by the controller, a volume of oil in an oil chamber of the shock strut, calculating, by the controller, a volume of gas in a primary chamber of the shock strut, and calculating, by the controller, a volume of oil leaked into the primary chamber of the shock strut.

FIELD

The present disclosure relates to landing gear, and more particularly,to systems and methods for monitoring shock struts.

BACKGROUND

Shock absorbing devices are used in a wide variety of vehicle suspensionsystems for controlling motion of the vehicle and its tires with respectto the ground and for reducing transmission of transient forces from theground to the vehicle. Shock absorbing struts are a common component inmost aircraft landing gear assemblies. Shock struts control motion ofthe landing gear, and absorb and damp loads imposed on the gear duringlanding, taxiing, braking, and takeoff.

A shock strut generally accomplishes these functions by compressing afluid within a sealed chamber formed by hollow telescoping cylinders.The fluid generally includes both a gas and a liquid, such as hydraulicfluid or oil. One type of shock strut generally utilizes an“air-over-oil” arrangement wherein a trapped volume of gas is compressedas the shock strut is axially compressed, and a volume of oil is meteredthrough an orifice. The gas acts as an energy storage device, similar toa spring, so that upon termination of a compressing force the shockstrut returns to its original length. Shock struts also dissipate energyby passing the oil through the orifice so that as the shock absorber iscompressed or extended, its rate of motion is limited by the dampingaction from the interaction of the orifice and the oil.

Performance of the shock strut assembly may degrade over time. Suchdegradation can cause damage to other components of the aircraft,including bearings of the landing gear assembly.

Functionality and performance of a landing gear shock strut depends oninternal gas and oil levels. Gas pressure and oil volume may bemaintained within a design envelope to ensure that the landing gearfunctionality is within an acceptable range.

SUMMARY

A monitoring system for a dual-stage, separated gas/fluid shock strut isdisclosed herein, in accordance with various embodiments. The monitoringsystem for a dual-stage, separated gas/fluid gas shock strut maycomprise a controller; and a tangible, non-transitory memory configuredto communicate with the controller, the tangible, non-transitory memoryhaving instructions stored thereon that, in response to execution by thecontroller, cause the controller to perform operations comprising:receiving, by the controller, a primary chamber temperature sensorreading; receiving, by the controller, a primary chamber pressure sensorreading; receiving, by the controller, a secondary chamber pressuresensor reading; receiving, by the controller, a shock strut strokesensor reading; determining, by the controller, a shock strut stroke atwhich a secondary chamber of the shock strut is activated; calculating,by the controller, a volume of oil in an oil chamber of the shock strut;calculating, by the controller, a primary chamber gas volume in aprimary chamber of the shock strut; and calculating, by the controller,a primary chamber oil leakage volume of oil leaked into the primarychamber of the shock strut.

In various embodiments, the instructions may cause the controller toperform further operations comprising calculating, by the controller, anumber of moles of gas in the primary chamber of the shock strut, basedupon at least the primary chamber gas volume. The instructions may causethe controller to perform further operations comprising: receiving, bythe controller, a secondary chamber temperature sensor reading;calculating, by the controller, a secondary chamber gas volume in thesecondary chamber; and calculating, by the controller, a volume of oilleaked into the secondary chamber of the shock strut, based upon anominal volume, based upon at least one of the secondary chamberpressure sensor reading, the secondary chamber temperature sensorreading, a displaced volume of the secondary chamber, and a total volumeof the secondary chamber. The instructions may cause the controller toperform further operations comprising calculating, by the controller, anumber of moles of gas in the secondary chamber of the shock strut,based upon at least the secondary chamber gas volume. The instructionscause the controller to perform further operations comprisingcalculating, by the controller, the displaced volume of the secondarychamber of the shock strut. The instructions may cause the controller toperform further operations comprising comparing, by the controller, thevolume of oil in the oil chamber with a plurality of threshold values,and issuing, by the controller, a servicing message, in response to thecomparing. The instructions may cause the controller to perform furtheroperations comprising comparing, by the controller, the number of molesof gas in the primary chamber with a plurality of threshold values, andissuing, by the controller, a servicing message, in response to thecomparing. The instructions may cause the controller to perform furtheroperations comprising comparing, by the controller, the number of molesof gas in the secondary chamber with a plurality of threshold values,and issuing, by the controller, a servicing message, in response to thecomparing. The instructions cause the controller to perform furtheroperations comprising comparing, by the controller, the volume of oilleaked into the primary chamber with a plurality of threshold values,and issuing, by the controller, a servicing message, in response to thecomparing. The instructions may cause the controller to perform furtheroperations comprising comparing, by the controller, the volume of oilleaked into the secondary chamber with a plurality of threshold values,and issuing, by the controller, a servicing message, in response to thecomparing. The controller may be in electronic communication with afirst pressure/temperature sensor for the primary chamber, a secondpressure/temperature sensor for a secondary chamber, and a strokesensor. The instructions may cause the controller to perform furtheroperations comprising adjusting the volume of oil in the oil chamber toa reference temperature. The instructions may cause the controller toperform further operations comprising calculating, by the controller, adeviation of the volume of oil in the primary chamber from a nominal oilvolume for the oil chamber.

A dual-stage, separated gas/fluid shock strut arrangement is disclosedherein in accordance with various embodiments. The dual-stage, separatedgas/fluid shock strut arrangement may comprise a dual-stage, separatedgas/fluid shock strut and a monitoring system. The dual-stage, separatedgas/fluid shock strut may comprise a strut cylinder, a strut pistonoperatively coupled to the strut cylinder, an oil chamber, a primary gaschamber, and a secondary gas chamber. The monitoring system may comprisea first pressure/temperature sensor mounted to the primary gas chamber,a second pressure/temperature sensor mounted to the secondary gaschamber, a stroke sensor mounted to the shock strut, a recorderconfigured to receive a plurality of sensor readings from at least oneof the first pressure/temperature sensor, the secondpressure/temperature sensor, and the stroke sensor, a landing detectorconfigured to detect a landing event based upon a stroke sensor readingreceived from the stroke sensor, and a health monitor configured todetermine a volume of oil in the oil chamber, a primary chamber gasvolume in the primary gas chamber, and a secondary chamber gas volume inthe secondary gas chamber.

In various embodiments, the monitoring system may further comprise atake-off detector configured to detect a take-off event based upon thestroke sensor reading received from the stroke sensor. The primary gaschamber may be separated from the oil chamber by a first separatorpiston and the secondary gas chamber is separated from the oil chamberby a second separator piston. The monitoring system may further comprisea counter configured to prevent at least one of the landing detector andthe take-off detector from receiving data from the recorder for apredetermined duration, and a data logger configured to receive datafrom the health monitor. The health monitor may calculate a shock strutstroke at which the secondary chamber of the shock strut is activatedbased upon a primary chamber pressure and a secondary chamber pressure.

A method for monitoring a dual-stage, separated gas/fluid shock strut isdisclosed herein, in accordance with various embodiments. The method maycomprise: receiving, by a controller, a primary chamber temperaturesensor reading; receiving, by the controller, a primary chamber pressuresensor reading; receiving, by the controller, a secondary chamberpressure sensor reading; receiving, by the controller, a secondarychamber temperature sensor reading; receiving, by the controller, ashock strut stroke sensor reading; determining, by the controller, ashock strut stroke at which a secondary chamber is activated;calculating, by the controller, a volume of oil in an oil chamber of theshock strut; calculating, by the controller, a primary chamber gasvolume in a primary gas chamber of the shock strut; calculating, by thecontroller, a number of moles of gas in the primary chamber of the shockstrut; calculating, by the controller, a volume of oil leaked into theprimary chamber of the shock strut; calculating, by the controller, asecondary chamber gas volume in a secondary chamber of the shock strut;calculating, by the controller, a volume of oil leaked into thesecondary chamber of the shock strut; and calculating, by thecontroller, a number of moles of gas in the secondary chamber, basedupon at least one of the secondary chamber pressure sensor reading, andthe secondary chamber temperature sensor reading.

In various embodiments, the method may further comprise: calculating, bythe controller, a displaced volume of the primary chamber; calculating,by the controller, a displaced volume of the secondary chamber; andissuing, by the controller, a servicing message.

The forgoing features and elements may be combined in variouscombinations without exclusivity, unless expressly indicated hereinotherwise. These features and elements as well as the operation of thedisclosed embodiments will become more apparent in light of thefollowing description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a functional schematic view of a dual-stage,separated gas/fluid shock strut at a stroke of zero (0) (or maximumextension), in accordance with various embodiments;

FIG. 2 illustrates a schematic view of the dual-stage, separatedgas/fluid shock strut of FIG. 1 at a secondary gas chamber activationstroke (S_(activation)), in accordance with various embodiments;

FIG. 3 illustrates a schematic view of the dual-stage, separatedgas/fluid shock strut of FIG. 1 at a maximum stroke (S_(max)) (fullycompressed position or zero extension), in accordance with variousembodiments;

FIG. 4A illustrates a schematic view of a dual-stage, separatedgas/fluid shock strut arrangement comprising the dual-stage, separatedgas/fluid shock strut of FIG. 2 and a monitoring system, in accordancewith various embodiments;

FIG. 4B illustrates a schematic view of the dual-stage, separatedgas/fluid shock strut arrangement of FIG. 4A, with a more detailed viewof the monitoring system, in accordance with various embodiments;

FIG. 4C illustrates a schematic view of a portion of the monitoringsystem of FIG. 4B with a take-off detector, in accordance with variousembodiments;

FIG. 5 illustrates a dynamic airspring curve of a primary gas chamberand a secondary gas chamber, in accordance with various embodiments;

FIG. 6 illustrates an algorithm for estimating a volume of oil in an oilchamber of the shock strut, in accordance with various embodiments;

FIG. 7 illustrates an algorithm for estimating a displacement volume ofa primary gas chamber, in accordance with various embodiments;

FIG. 8 illustrates an algorithm for estimating a volume of oil leakedinto a secondary gas chamber, in accordance with various embodiments;and

FIG. 9 illustrates a method for monitoring a dual-stage, separatedgas/fluid shock strut, in accordance with various embodiments.

The subject matter of the present disclosure is particularly pointed outand distinctly claimed in the concluding portion of the specification. Amore complete understanding of the present disclosure, however, may bestbe obtained by referring to the detailed description and claims whenconsidered in connection with the drawing figures, wherein like numeralsdenote like elements.

DETAILED DESCRIPTION

The detailed description of exemplary embodiments herein makes referenceto the accompanying drawings, which show exemplary embodiments by way ofillustration. While these exemplary embodiments are described insufficient detail to enable those skilled in the art to practice thedisclosure, it should be understood that other embodiments may berealized and that logical changes and adaptations in design andconstruction may be made in accordance with this disclosure and theteachings herein without departing from the spirit and scope of thedisclosure. Thus, the detailed description herein is presented forpurposes of illustration only and not of limitation.

System program instructions and/or controller instructions may be loadedonto a tangible, non-transitory, computer-readable medium (also referredto herein as a tangible, non-transitory, memory) having instructionsstored thereon that, in response to execution by a controller, cause thecontroller to perform various operations. The term “non-transitory” isto be understood to remove only propagating transitory signals per sefrom the claim scope and does not relinquish rights to all standardcomputer-readable media that are not only propagating transitory signalsper se. Stated another way, the meaning of the term “non-transitorycomputer-readable medium” and “non-transitory computer-readable storagemedium” should be construed to exclude only those types of transitorycomputer-readable media which were found in In Re Nuijten to falloutside the scope of patentable subject matter under 35 U.S.C. § 101.

A shock strut gas pressure and stroke in static condition may bemeasured and any deviation from the shock strut theoretical staticairspring curve typically may be compensated by re-servicing the shockstrut with gas. Such an approach may be taken to reduce maintenance timeassociated with just adding gas to the shock strut. However, saidapproach assumes the deviation from static airspring curve is solely dueto gas loss and therefore could overlook an oil leak in the system.

Aircraft landing gear systems in accordance with the present disclosuremay comprise a shock strut. A shock strut may comprise various fluidssuch as oil and gas. Performance of the shock strut may be evaluated bymonitoring aspects of the shock strut, including primary chamber gastemperature, primary chamber gas pressure, secondary chamber gastemperature, secondary chamber gas pressure, and shock strut stroke ofthe shock strut at various points during operation of the aircraft.Stroke may refer to a shock strut piston position relative to a shockstrut cylinder.

A monitoring system, as provided herein, may comprise two integratedpressure/temperature sensors installed on the primary and secondary gaschambers of a dual-stage, separated gas/fluid shock strut, a strokesensor that directly or indirectly measures the shock strut stroke, andan electronic control unit that executes a monitoring algorithm. Themonitoring algorithm may use transient gas pressure and gas temperatureduring landing or takeoff and quantifies the oil and gas levels in theshock strut. Moreover, the monitoring system may estimate oil leakageinto the gas chamber. The monitoring algorithm may issue a servicingmessage based on the shock strut estimated fluid and gas levels.

Because oil and gas levels may be determined independently, said levelscan be used for diagnostic and prognostic purposes. The rate of oil orgas loss may be used to schedule future servicing.

The following nomenclature in table 1 and table 2 corresponds to variousequations and parameters described in the present disclosure:

TABLE 1 Measurements Measurements {circumflex over (P)}_(primary)Primary chamber gas pressure sensor reading {circumflex over(T)}_(primary) Primary chamber gas temperature sensor reading{circumflex over (P)}_(secondary) Secondary chamber gas pressure sensorreading {circumflex over (T)}_(secondary) Secondary chamber gastemperature sensor reading Ŝ Shock strut stroke sensor reading{circumflex over (P)}_(primary) (0) Primary chamber pressure sensorreading at a shock strut stroke of 0 (or near 0) (e.g., 25% of maximumstroke or less) {circumflex over (T)}_(primary) (0) Primary chambertemperature sensor reading at a shock strut stroke of 0 (or near 0){circumflex over (P)}_(secondary) (0) Secondary chamber pressure sensorreading at a shock strut stroke of 0 (or near 0) {circumflex over(T)}_(secondary) (0) Secondary chamber temperature sensor reading at ashock strut stroke of 0 (or near 0) {circumflex over (P)}_(primary) (S)Primary chamber pressure sensor reading at a shock strut stroke of S{circumflex over (P)}_(primary) (S_(activation)) Primary chamberpressure sensor reading at the secondary chamber activation stroke{circumflex over (P)}_(primary,max) Maximum primary chamber pressureduring landing Ŝ_(primary,max) Shock strut stoke at which primarychamber reaches its maximum level {circumflex over (P)}_(secondary@Ŝ)_(primary,max) Secondary gas pressure at the shock strut stroke ofŜ_(primary,max)

TABLE 2 Algorithm Internal Parameters Algorithm Internal ParametersA_(p) Shock strut piston area S_(activation) Estimated activation strokeof the secondary chamber {circumflex over (V)}_(oil) Optimizationalgorithm guess for oil volume V_(oil)(0) Estimated oil volume at ashock strut stroke of 0 {circumflex over (P)}_(primary) (S_(activation))Calculated Primary chamber pressure at the secondary chamber activationstroke V_(tot) Total internal volume of the shock strut in the fullyextended position V_(secondary) _(—) _(chamber) _(—) _(nom) Secondarychamber nominal internal volume at the shock strut stroke of 0V_(primary) _(—) _(chamber) (0) Estimated primary chamber internalvolume at the shock strut stroke of 0 (or near 0) (e.g., 25% of maximumstroke or less) Z Nitrogen compressibility factor R Ideal gas constant βOil bulk modulus V_(oil) (Ŝ_(primary,max)) Oil volume at the shock strutstroke of Ŝ_(primary,max) V_(primary+secondary) (Ŝ_(primary,max)) Totalvolume of primary and secondary chambers at the shock strut stroke ofŜ_(primary,max) Δ{circumflex over (V)}_(primary chamber) Optimizationalgorithm guess for primary chamber displacement volume {circumflex over(P)}_(primary,max) Calculated primary chamber pressure at the shockstrut stroke of Ŝ_(primary,max) {circumflex over (V)}_(primary) _(—)_(chamber) (Ŝ_(primary,max)) Calculated primary chamber volume at theshock strut stroke of Ŝ_(primary,max) V_(primary) _(—) _(chamber) _(—)_(dead) Primary chamber nominal dead volume V_(primary) _(—) _(chamber)_(—) _(leakage) Calculated oil leakage volume into the primary chamber{circumflex over (V)}_(secondary) _(—) _(chamber) (Ŝ_(primary,max))Calculated secondary chamber volume at the shock strut stroke ofŜ_(primary,max) Δ{circumflex over (V)}_(secondary chamber) Calculatedsecondary chamber displacement volume V_(secondary) _(—) _(chamber) _(—)_(leakage) Calculated oil leakage volume into the secondary chamber{circumflex over (P)}_(secondary@Ŝ) _(primary,max) Estimated secondarychamber pressure at the shock strut stroke of Ŝ_(primary,max)n_(primary) _(—) _(chamber) Primary chamber calculated number of molesof gas n_(secondary) _(—) _(chamber) Secondary chamber calculated numberof moles of gas T_(ref) Reference temperature dT Numerical integrationstep α Oil thermal expansion coefficient V_(oil nom) Nominal oil volumeV_(oil@T) _(ref) Oil volume at T_(ref) V_(threshold) Oil volumethreshold P_(primary) _(—) _(nom) Primary chamber nominal pressureV_(primary) _(—) _(chamber) _(—) _(nom) Primary chamber nominal volumen_(primary) _(—) _(chamber) _(—) _(nominal) Primary chamber nominalnumber of moles n_(primary,threshold) Primary chamber thresholdP_(secondary) _(—) _(nom) Secondary chamber nominal pressureV_(secondary) _(—) _(chamber) _(—) _(nom) Secondary chamber nominalvolume n_(secondary) _(—) _(chamber) _(—) _(nominal) Secondary chambernominal number of moles n_(secondary,threshold) Secondary chamberthreshold S_(min,takeoff) Minimum shock strut stroke parameter fordetermining a takeoff event S_(max,takeoff) Maximum shock strut strokeparameter for determining a takeoff event

In various embodiments, a monitoring system for a dual-stage, separatedgas/fluid shock strut is provided herein. A functional schematic view ofsuch a shock strut is presented in FIG. 1.

With reference to FIG. 1, a dual-stage, separated gas/fluid shock strut(shock strut) 100 is illustrated, in accordance with variousembodiments. Shock strut 100 may comprise a strut cylinder 110 and astrut piston 120. Strut piston 120 may be operatively coupled to strutcylinder 110 as described herein. Strut cylinder 110 may be configuredto receive strut piston 120 in a manner that allows the two componentsto telescope together and absorb and dampen forces transmitted thereto.In various embodiments, a liquid, such as a hydraulic fluid and/or oilmay be located within strut cylinder 110. Further, a gas, such asnitrogen or air, may be located within strut cylinder 110. Strutcylinder 110 and strut piston 120 may, for example, be configured toseal such that fluid contained within strut cylinder 110 is preventedfrom leaking as strut piston 120 translates relative to strut cylinder110.

Shock strut 100 may consist of a low pressure, primary gas chamber 130in which gas is contained. In this regard, a volume of gas (alsoreferred to herein as a primary chamber gas volume) 131 may be containedwithin primary gas chamber 130. Shock strut 100 may further consist of ahigh pressure, secondary gas chamber 140. In this regard, a volume ofgas 141 (also referred to herein as a secondary chamber gas volume) maybe contained within secondary gas chamber 140. The volume of gas 131 maybe at a lower pressure than the volume of gas 141 when shock strut 100is in the fully extended position (i.e., at a shock strut stroke 193 ofzero). Primary gas chamber 130 may be located at a first end 191 ofshock strut 100. First end 191 may be the bottom of shock strut 100.Secondary gas chamber 140 may be located at a second end 192 of shockstrut 100. Second end 192 may be the top of shock strut 100.

Shock strut 100 may further consist of an oil chamber 150. In thisregard, a volume of oil (also referred to herein as an oil chamber oilvolume) 151 may be contained within oil chamber 150. Primary gas chamber130 may be separated from oil chamber 150 via a separator piston (alsoreferred to herein as a first separator piston) 132. Secondary gaschamber 140 may be separated from oil chamber 150 via a separator piston(also referred to herein as a second separator piston) 142. Separatorpiston 142 may translate within secondary gas chamber 140. FIG. 1illustrates separator piston 142 at a minimum compression stroke (alsoreferred to as being “bottomed out”). Stated differently, with shockstrut 100 in the fully extended position, separator piston 142 may belocated in a position such that the volume of secondary gas chamber 140is at its maximum value. When separator piston 142 is bottomed out, itmay be mechanically prevented from translating towards first end 191.

Shock strut 100 may further consist of an orifice plate 114. Orificeplate 114 may be located in oil chamber 150. Shock strut 100 maycomprise an oil charge port 102 in fluid communication with oil chamber150. Shock strut 100 may comprise an oil bleed port 104 in fluidcommunication with oil chamber 150. Shock strut 100 may comprise aprimary chamber gas charge port 135 in fluid communication with primarygas chamber 130. Shock strut 100 may comprise a secondary chamber gascharge port 145 in fluid communication with secondary gas chamber 140.

In various embodiments, shock strut 100 may be installed onto a landinggear of an aircraft. During a landing event, shock strut 100 may becompressed wherein strut piston 120 translates into strut cylinder 110.During the landing, the shock strut may initially function as asingle-stage, separated gas/fluid shock strut by metering oil throughorifice plate 114 and compressing the volume of gas 131 in primary gaschamber 130. The compression of primary gas chamber 130 may continueuntil the pressure in primary gas chamber 130 is equal to or greaterthan the pressure of the volume of gas 141 within secondary gas chamber140. As illustrated in FIG. 2, this occurs at a shock strut stroke 194,(i.e., S_(activation)) of between zero and the maximum shock strutstroke, S_(max). Separator piston 132 may translate towards first end191 as shock strut 100 is compressed. Once the secondary gas chamber 140is activated, further compression of the shock strut may compress thevolume of gas 141 in the secondary gas chamber 140, as illustrated inFIG. 3. FIG. 3 illustrates shock strut 100 in a fully compressedposition, or at a maximum shock strut stroke 195 (i.e., S_(max)). FIG. 3illustrates separator piston 132 at a maximum compression stroke (alsoreferred to as being “bottomed out”). When separator piston 132 isbottomed out, it may be mechanically prevented from translating towardsfirst end 191.

With reference to FIG. 4A and FIG. 4B, a dual-stage, separated gas/fluidshock strut arrangement (shock strut arrangement) 400 is illustrated, inaccordance with various embodiments. Shock strut arrangement 400 mayinclude shock strut 100 and a monitoring system 200. Monitoring system200 may comprise various sensing elements. Monitoring system 200 maycomprise an integrated pressure/temperature sensor (also referred toherein as a first pressure/temperature sensor) 202 installed on theprimary gas chamber 130 to measure gas pressure and temperature withinprimary gas chamber 130. Although described herein as an integratedpressure/temperature sensor 202, it is contemplated herein that aseparate pressure sensor and temperature sensor may be used in place ofintegrated pressure/temperature sensor 202. Monitoring system 200 maycomprise an integrated pressure/temperature sensor (also referred toherein as a second pressure/temperature sensor) 204 installed on thesecondary gas chamber 140 to measure gas pressure and temperature withinsecondary gas chamber 140. Although described herein as an integratedpressure/temperature sensor 204, it is contemplated herein that aseparate pressure sensor and temperature sensor may be used in place ofintegrated pressure/temperature sensor 204. In this regard, the term“pressure/temperature sensor” as used herein, may refer to either anintegrated pressure/temperature sensor or to separate pressure andtemperature sensors. Monitoring system 200 may comprise a positionsensor (also referred to herein as a stroke sensor) 206 configured todirectly or indirectly measure the shock strut stroke 258 (Ŝ). In thisregard, the sensors may measure various parameters and providemeasurements to monitoring system 200.

Integrated pressure/temperature sensor 202 may measure primary chambergas pressure 250 ({circumflex over (P)}_(primary)), and primary chambergas temperature 252 ({circumflex over (T)}_(primary)). Integratedpressure/temperature sensor 204 may measure secondary chamber gaspressure 254 ({circumflex over (P)}_(secondary)), and secondary chambergas temperature 256 ({circumflex over (T)}_(secondary)). Stroke sensor206 may measure shock strut stroke 258 (Ŝ). Primary chamber gaspressure, {circumflex over (P)}_(primary), primary chamber gastemperature, {circumflex over (T)}_(primary), secondary chamber gaspressure, {circumflex over (P)}_(secondary), secondary chamber gastemperature, T_(secondary), and shock strut stroke, S may be referred toherein as sensor readings (e.g., primary chamber gas pressure sensorreading). In various embodiments, {circumflex over (T)}_(primary) and{circumflex over (T)}_(secondary) may be used interchangeably.

Monitoring system 200 may be devised assuming that the sensors comprisea minimum sampling frequency of between 10 Hz and 1000 Hz in accordancewith various embodiments, between 60 Hz and 200 Hz in accordance withvarious embodiments, or about 100 Hz in accordance with variousembodiments, wherein the term “about” in this regard may mean±20 Hz.

With reference to FIG. 4A, monitoring system 200 may comprise acontroller 201 and a tangible, non-transitory memory 208 configured tocommunicate with the controller 201. The tangible, non-transitory memory208 may have instructions stored thereon that, in response to executionby the controller 201, cause the controller 201 to perform variousoperations as described herein. Monitoring system 200 may comprise avisual display 270. Visual display 270 may be in electroniccommunication with controller 201. As described herein, controller 201may issue or send a servicing message 272. Servicing message 272 may bedisplayed on visual display 270. In various embodiments, servicingmessage 272 may comprise an indication of a quantity of oil or gas inshock strut 100. In various embodiments, servicing message 272 maycomprise a current and/or a voltage signal. Controller 201 may be inelectronic communication with integrated pressure/temperature sensor 202and integrated pressure/temperature sensor 204. FIG. 4B illustratesmonitoring system 200 in further detail.

In various embodiments, with reference to FIG. 4B, monitoring system 200may comprise a recorder 210, a landing detector 220, a counter 225, ahealth monitor 230, and/or a data logger 240. Recorder 210, landingdetector 220, counter 225, health monitor 230, and/or data logger 240may comprise instructions stored in a tangible, non-transitory memory208 (see FIG. 4A). Recorder 210, landing detector 220, counter 225,health monitor 230, and/or data logger 240 may be implemented on one ormore controllers (e.g., controller 201 of FIG. 4A). In this regard,controller 201 (see FIG. 4A) may comprise one or more controllers. Forexample, a first controller (e.g., recorder 210) may receive sensorinformation and a second controller (e.g., health monitor 230) mayperform the calculations as described herein.

In various embodiments, recorder 210 may receive primary chamber gaspressure 250 ({circumflex over (P)}_(primary)), primary chamber gastemperature 252 ({circumflex over (T)}_(primary)), secondary chamber gaspressure 254 ({circumflex over (P)}_(secondary)), secondary chamber gastemperature 256 ({circumflex over (T)}_(secondary)), and shock strutstroke 258 (Ŝ), and record them in an array that keeps the readings fora pre-determined length of time, such as 15 seconds for example. A newset of recordings may be added to the top of the array and the oldestset of data may be eliminated from the bottom of the array to keep thelength of the array constant. At any instant, recorder 210 may exportthe array which comprises the latest set of data recorded over thepre-determined length of time to the landing detector 220. Recorder 210may receive the sensor readings in real-time or at a later time.

At the startup when the length of the data array 214 is not equivalentto 15 seconds (tunable parameter), recorder 210 may send a falsedetection state signal 212 to landing detector 220 to prevent landingdetector 220 from using the incomplete array. Once 15 seconds (tunableparameter) of measurement is available, the detection state signal 212may turn true to allow landing detector 220 to use the measurements.

In various embodiments, landing detector 220 may receive the array ofdata 214 and check the array against the following set of criteria:first, that the minimum stroke in the array is smaller than a minimumdimension, such as 0.2″ (tunable parameter), second, that the maximumstroke in the array is bigger than a maximum dimension, such as 5″(tunable parameter), third, that the stroke for the first five (5)seconds of the array is less than the minimum dimension, and fourth,that the maximum stroke in the first ten (10) seconds (tunableparameter) of the array is bigger than a threshold dimension, such as 4″(tunable parameter).

The first two criteria may ensure that the set of data is associated toa landing or a takeoff or any other event that has caused the shockstrut to travel between 0.2″ to 5″ (tunable parameters). The thirdcriterion may ensure that the set of data is associated to a landing asin the first five (5) seconds the shock strut has been fully extended.The fourth criterion may ensure that the selected set of data alsoincludes 5 seconds of measurement after compression. It is contemplatedherein that the algorithm parameters may be tuned according to variousembodiments, for example may be tuned up further. If the data array 214meets all these criteria, it is categorized as a landing event andexported to health monitor 230. A signal 227 may also be sent to thehealth monitor 230 indicating that the data array 214 meets all of theabove criteria. A counter 225 may also be started to prevent landingdetector 220 from receiving any new array of measurements for apredetermined duration, such as five (5) minutes (tunable parameter).This may relax the need for a high speed processor for health monitoringpurposes. If the data array 214 does not meet all the criteria, landingdetector 220 may disregard the array and wait for the new array of data.

In various embodiments, health monitor 230 may receive the array of data214 that includes various sensor measurements. In various embodiments,the sensor measurements may include primary chamber gas pressure 250({circumflex over (P)}_(primary)), primary chamber gas temperature 252({circumflex over (T)}_(primary)), secondary chamber gas pressure 254({circumflex over (P)}_(secondary)), secondary chamber gas temperature256 ({circumflex over (T)}_(secondary)), and/or shock strut stroke 258(Ŝ). The array of data 214 may be received by health monitor 230 for apre-determined length of time, such as 15 seconds, for example.

With reference to FIG. 4C, a portion of monitoring system 200 isillustrated with a take-off detector 260. In this regard, in addition tolanding detector 220, monitoring system 200 may further comprisetake-off detector 260. It may be desirable to ensure that sensorreadings are available when shock strut 100 is in the fully extendedposition, as illustrated in FIG. 1. Thus, take-off detector 260 may beprovided to detect a take-off event. After take-off, shock strut 100 maybe in a fully extended position. In this regard, sensor readings takenafter take-off may comprise values corresponding to a shock strut strokeof zero (0). In this regard, monitoring system 200 may use sensorreadings from data array 264 for calculations which use datacorresponding to a shock strut stroke of zero (0), as described herein.

Take-off detector 260 may operate similar to landing detector 220, butusing different criteria to examine stroke sensor readings to determinethe take-off event. A data array 264 may be sent from recorder 210 totake-off detector 260. Similarly, data array 264 may be sent to healthmonitor 230. Data array 264 may be similar to data array 214 asdescribed in FIG. 4B. In this regard, at the startup when the length ofthe data array 264 is not equivalent to a predetermined duration, suchas 15 seconds for example, recorder 210 may send a false detection statesignal 212 to take-off detector 260 to prevent take-off detector 260from using the incomplete array. Once the predetermined duration ofmeasurement is available, the detection state signal 212 may turn trueto allow take-off detector 260 to use the measurements in the data array264.

In various embodiments, take-off detector 260 may receive the array ofdata 264 and check the array against the following set of criteria:first, that the minimum stroke in the array is less than a minimumdimension (i.e., S_(min,takeoff)), such as 0.2″ (tunable parameter),second, that the maximum stroke in the array is greater than a maximumdimension (i.e., S_(max,takeoff)), such as 5″ (tunable parameter),third, that the stroke for the first five (5) seconds of the array isgreater than the maximum dimension (i.e., S_(max,takeoff)), and fourth,that the minimum stroke in the first ten (10) seconds (tunableparameter) of the array is less than the minimum dimension (i.e.,S_(min,takeoff)).

The first two criteria may ensure that the set of data is associated toa landing or a takeoff or any other event that has caused the shockstrut to travel between 0.2″ to 5″ (tunable parameters). The thirdcriterion may ensure that the set of data is associated to a takeoffbecause in the first five (5) seconds of data the shock strut iscompressed to a shock strut stroke greater than S_(max,takeoff). Thefourth criterion may ensure that the selected set of data also includes5 seconds of measurement after takeoff. It is contemplated herein thatthe algorithm parameters may be tuned according to various embodiments.If the data array 264 meets all these criteria, it is categorized as atake-off event and exported to health monitor 230. A signal 267 may alsobe sent to the health monitor 230 indicating that the data array 264meets all of the above criteria. A counter 265 may also be started toprevent take-off detector 260 from receiving any new array ofmeasurements for a predetermined duration, such as five (5) minutes(tunable parameter). This may relax the need for a high speed processorfor health monitoring purposes. If the data array 264 does not meet allthe criteria, take-off detector 260 may disregard the array and wait forthe new array of data.

Pressure Loss Check:

With combined reference to FIG. 4B and FIG. 5, the measured primarychamber gas pressure 250 at a shock strut stroke of zero (i.e.,{circumflex over (P)}_(primary)(0)) and the measured secondary chambergas pressure 254 at a shock strut stroke of zero (i.e., {circumflex over(P)}_(secondary)(0)) may be compared to determine that a major pressureloss has not occurred in the secondary gas chamber 140. In this regard,monitoring system 200 may determine that the following equation holds:{circumflex over (P)} _(secondary)(0)≥{circumflex over (P)}_(primary)(0)+σ  Eq. (1)where σ is a predetermined value, such as 100 psi (689.476 kPa) forexample. σ may be selected to ensure that the difference between thesecondary chamber pressure and the primary chamber pressure is above thesensor measurement error range. If Eq. 1 is false, the monitoring system200 may generate a message indicating that there has been a major lossof pressure in the secondary gas chamber 140 and that the algorithmcannot be executed. If Eq. 1 is true, then monitoring system 200 mayperform the following operations, as described herein.Oil Volume Determination:

In various embodiments, the oil volume 151 may be determined via healthmonitor 230. With combined reference to FIG. 4B and FIG. 5, at the onsetof compression of the shock strut 100, the pressure within primary gaschamber 130 is less than the pressure within secondary gas chamber 140.As compression continues, the primary chamber gas pressure increases andmay exceed the secondary chamber gas pressure. Once the primary chambergas pressure exceeds the secondary chamber gas pressure, the secondarygas chamber 140 may be activated and further compression of shock strut100 may cause compression of both primary gas chamber 130 and secondarygas chamber 140. Monitoring system 200 may utilize the measured dynamicairspring curve 502 of the primary gas chamber 130, before secondarychamber activation, to determine volume of oil 151 in oil chamber 150.In the first stage of compression, the shock strut functions as asingle-stage, separated gas/fluid shock strut with a known initialinternal volume.

In this regard, monitoring system 200 may use dynamic airspring curve502 of primary gas chamber 130 and the secondary chamber inflationpressure (i.e., {circumflex over (P)}_(secondary)(0)) measured at ashock strut stroke of zero (or a shock strut stroke near zero such as ashock strut of 25% of the maximum shock strut stroke or less) todetermine the shock strut stroke at which the secondary chamber isactivated (i.e., S_(activation)). With focus on FIG. 5, it is noted thatthe activation stroke 512 of the secondary chamber (i.e.,S_(activation)) is the maximum shock strut stroke at which the primarychamber pressure is less than or equal to the secondary chamberinflation pressure. That is,S _(activation)={max S|{circumflex over (P)} _(primary)(Ŝ)≤{circumflexover (P)} _(secondary)(0)}  Eq. (2)

Once the activation stroke 512 of the secondary chamber is determined,the primary chamber inflation pressure in the fully extended position(i.e., {circumflex over (P)}_(primary)(0)), the primary chambertemperature in the fully extended position (i.e., {circumflex over(T)}_(primary)(0)), the activation stroke 512 of the secondary chamber(i.e., S_(activation)), and the primary chamber pressure atS_(activation) (i.e., {circumflex over (P)}_(primary)(S_(activation)))may be stored for use by monitoring system 200 to determine the volumeof oil 151.

With combined reference to FIG. 4B and FIG. 6, monitoring system 200 mayuse a dynamic airspring model 610, along with a numerical optimizationmethod 620 to estimate the volume of the oil 151 in oil chamber 150. Inthis regard, FIG. 6 illustrates an algorithm 600 for estimating the oilvolume 151 in oil chamber 150. Dynamic airspring model 610 may generatean accurate estimation of transient gas pressure for a single-stage,separated gas/fluid shock strut for a displacement volume, if oilvolume, initial temperature, initial gas pressure, and initial totalinternal volume are known. However, since the oil volume is required forthe model to work, the algorithm 600 provides an initial estimate forthe volume of oil (i.e., V _(oil)), computes the pressure atS_(activation), and then compares said pressure with the measuredpressure at S_(activation). Stated differently, health monitor 230 mayuse the primary chamber inflation pressure in the fully extendedposition (i.e., {circumflex over (P)}_(primary)(0)), the primary chambertemperature in the fully extended position (i.e., {circumflex over(T)}_(primary)(0)), the displacement volume of the primary gas chamber130 at S_(activation) (i.e., S_(activation)*A_(p)), the total volume ofthe primary gas chamber 130 and the oil chamber 150 (i.e.,V_(tot)−V_(secondary) _(_) _(chamber) _(_) _(nom)), and an initialestimate for the volume of the oil in oil chamber 150 (i.e., V _(oil)),for example 10 cubic inches (163.87 cm³), and may compute the primarychamber gas pressure at S_(activation).

A gradient free, single-variable, numerical optimization technique, suchas Bisection or Line Search methods may be used to adjust the initialestimate for the oil volume so that the difference between the measuredprimary gas chamber pressure at S_(activation) (i.e., {circumflex over(P)}_(primary)(S_(activation))), (see FIG. 5) and the estimated primarygas chamber pressure at S_(activation) (i.e., P_(primary)(S_(activation))) is minimized. In various embodiments, thenominal value of the oil volume may be used as the initial estimate forthe oil volume, which may improve the optimization convergence speed. Ablock diagram of the algorithm 600, described above, for shock strut oilvolume determination is provided in FIG. 6.

The optimization loop may continue until the absolute difference betweenthe estimated pressure and the measured pressure at S_(activation) isless than or equal to a pre-determined threshold as follows:|{circumflex over (P)} _(primary)(S _(activation))− P _(primary)(S_(activation))|≤Threshold 1  Eq. (3)

When equation 3 is satisfied, the last estimate for the oil volume(i.e., V _(oil)) may be recorded as the oil volume 151 inside the oilchamber 150 at the shock strut stroke of zero. That is:V _(oil)(0)= V _(oil)  Eq. (4)Primary Chamber Gas Level Determination:

Once the oil volume in the fully extended position is determined, theprimary chamber gas volume in the fully extended position may bedetermined as follows:V _(primary) _(_) _(chamber)(0)=V _(tot) −V _(secondary) _(_) _(chamber)_(_) _(nom) −V _(oil)(0)  Eq. (5)where V_(tot) is the total internal volume of the shock strut in thefully extended position and V_(secondary) _(_) _(chamber) _(_) _(nom) isthe nominal volume of the secondary chamber when shock strut 100 is inthe fully extended position (see FIG. 1).

The number of moles of gas in the primary gas chamber 130 of the shockstrut 100 may then be computed using the following equation:

$\begin{matrix}{n_{{primary}\_{chamber}} = \frac{{{\hat{P}}_{primary}(0)} \times {V_{{primary}\_{chamber}}(0)}}{R \times {{\hat{T}}_{primary}(0)} \times {Z\left( {{{\hat{P}}_{primary}(0)},{{\hat{T}}_{primary}(0)}} \right)}}} & {{Eq}.\mspace{14mu}(6)}\end{matrix}$where R is the ideal gas constant and Z({circumflex over(P)}_(primary)(0), {circumflex over (T)}_(primary)(0)) is the nitrogencompressibility factor. The computed number of moles of gas in theprimary gas chamber 130 may be then logged in the data logger 240.Although {circumflex over (P)}_(primary)(0) and {circumflex over(T)}_(primary)(0) are described herein as being measured during alanding event, it is contemplated herein that they may also be recordedafter a takeoff event. In various embodiments, {circumflex over(T)}_(primary)(0) and {circumflex over (T)}_(secondary)(0) at any pointduring a take-off event (e.g., detected by takeoff detector 260 withmomentary reference to FIG. 4C) or a landing event may be used. In thisregard, {circumflex over (P)}_(primary)(0) and {circumflex over(T)}_(primary)(0) are primary chamber gas pressure and temperature,respectively, when the shock strut 100 is in the fully extended position(or within 25% of the fully extended position) recorded either during alanding event or a takeoff event. It is noteworthy that instead ofprimary chamber temperature, {circumflex over (T)}_(primary)(0), thesecondary chamber temperature, {circumflex over (T)}_(secondary)(0), maybe used to calculate the number of moles of gas in the primary gaschamber 130.Primary Chamber Oil Leakage Volume Determination:

Depending on the aircraft's sink-rate, dynamic weight on the landinggear and the shock strut internal fluid levels the primary chamber mayor may not reach a maximum compression stroke (e.g., separator piston132 may “bottom out”) during a landing event. If the primary chamberdoes not reach a maximum compression stroke during the landing even, themaximum pressure achieved in the primary chamber will be equal to themaximum pressure in the secondary chamber at the maximum compressionstroke. If the primary chamber reaches the maximum compression stroke,the secondary chamber pressure continues to increase while the primarychamber pressure drops due to thermal losses. Under both conditions, amaximum pressure value for the primary chamber can be found. Moreover,under both conditions, the secondary chamber pressure will be nearlyequal to the primary chamber pressure when the primary chamber pressurereaches its maximum value.

In various embodiments, with combined reference to FIG. 4B and FIG. 5,monitoring system 200 may determine the maximum pressure (also referredto herein as the maximum primary chamber pressure) of primary gaschamber 130 during landing (i.e., {circumflex over (P)}_(primary,max))and the shock strut stroke associated with said pressure (i.e.,Ŝ_(primary,max)). Once {circumflex over (P)}_(primary,max) andŜ_(primary,max) are determined, the total volume of the primary andsecondary chambers at Ŝ_(primary,max) may be determined, as follows:

$\begin{matrix}{\mspace{79mu}{{{V_{oil}\left( {\hat{S}}_{{primary},\max} \right)} = {{V_{oil}(0)} \times \left( {1 - \frac{{\hat{P}}_{{primary},\max} - {{\hat{P}}_{primary}(0)}}{\beta}} \right)}}\mspace{79mu}{and}}} & {{Eq}.\mspace{14mu} 7} \\{{V_{{primary} + {secondary}}\left( {\hat{S}}_{{primary},\max} \right)} = {V_{tot} - {A_{p} \times {\hat{S}}_{{primary},\max}} - {V_{oil}\left( {\hat{S}}_{{primary},\max} \right)}}} & {{Eq}.\mspace{14mu} 8}\end{matrix}$where β is the oil bulk modulus, V_(tot) is the total internal volume ofthe shock strut in the fully extended position, and A_(p) is the pistonarea. V_(primary+secondary)(Ŝ_(primary,max)) is the total volume of theprimary and secondary gas chambers at the stroke at which the primarychamber pressure reaches its maximum level during landing.

With combined reference to FIG. 4B and FIG. 7, monitoring system 200 mayuse a dynamic airspring model 710, along with a numerical optimizationmethod 720 to estimate the displacement volume of the primary chamber atŜ_(primary,max). In this regard, FIG. 7 illustrates an algorithm 700 forestimating the displacement volume of the primary chamber atŜ_(primary,max). Dynamic airspring model 710 may generate an accurateestimation of transient gas pressure for a single-stage, separatedgas/fluid shock strut for a displacement volume, if oil volume, initialtemperature, initial gas pressure, and initial total internal volume areknown. However, since the displacement volume is required for the modelto work, the algorithm 700 provides an initial estimate for thedisplacement volume (i.e., ΔV _(primary) _(_) _(chamber)), computes thepressure at Ŝ_(primary,max), and then compares said pressure with themeasured pressure at Ŝ_(primary,max). Stated differently, health monitor230 may use the measured primary chamber inflation pressure in the fullyextended position (i.e., {circumflex over (P)}_(primary)(0)), theprimary chamber temperature in the fully extended position (i.e.,{circumflex over (T)}_(primary)(0)), the initial estimate for thedisplacement volume of the primary gas chamber 130 (i.e., ΔV _(primary)_(_) _(chamber)), the total volume of the primary gas chamber 130,calculated by Eq. 5 (i.e., V _(primary) _(_) _(chamber) (0)), and an oilvolume of zero, and may compute the primary chamber gas pressure atŜ_(primary,max).

A gradient free, single-variable, numerical optimization technique, suchas Bisection or Line Search methods may be used to adjust the initialestimate for displacement volume so that the difference between themeasured primary gas chamber pressure at Ŝ_(primary,max) (i.e.,{circumflex over (P)}_(primary,max), (see FIG. 5) and the estimatedprimary gas chamber pressure at Ŝ_(primary,max) (i.e., {circumflex over(P)}_(primary,max) is minimized.

The optimization loop may continue until the absolute difference betweenthe estimated pressure and the measured pressure at S_(primary,max) isless than or equal to a pre-determined threshold as follows:|{circumflex over (P)} _(primary,max) −P _(primary,max)|≤Threshold2  Eq. (9)

When equation 9 is satisfied, the last estimate for the displacementvolume of the primary gas chamber (i.e., ΔV_(primary) _(_) _(chamber))may be recorded and the gas volume in the primary gas chamber 130 atŜ_(primary,max) may be determined as follows:V _(primary) _(_) _(chamber)(Ŝ _(primary,max))=V _(primary) _(_)_(chamber) −ΔV _(primary) _(_) _(chamber)  Eq. (10)

If V_(primary) _(_) _(chamber)(Ŝ_(primary,max)) is larger than theprimary chamber dead volume (i.e., V_(primary) _(_) _(chamber) _(_)_(dead)), no conclusion may be made regarding the possible oil leakageinto the primary chamber. If V_(primary) _(_)_(chamber)(S_(primary,max)) is smaller or equal to the primary chamberdead volume, then the volume of oil leakage into the primary chamber(also referred to herein as a primary chamber oil leakage volume) isestimated as follows:If V _(primary) _(_) _(chamber)(Ŝ _(primary,max))≤V _(primary) _(_)_(chamber) _(_) _(dead) →V _(primary) _(_) _(chamber) _(_) _(leakage) =V_(primary) _(_) _(chamber) _(_) _(dead) −V _(primary) _(_) _(chamber)(Ŝ_(primary,max))  Eq. (11)Secondary Chamber Gas Level and Oil Leakage Volume Determination:

In various embodiments, with reference to FIG. 4B, the secondary gaschamber 140 gas level may be determined. In this step, monitoring system200 may compute the volume of the secondary gas chamber 140 at thestroke of Ŝ_(primary,max) as follow:V _(secondary) _(_) _(chamber)(S _(primary,max))=V_(primary+secondary)(Ŝ _(primary,max))−V _(primary) _(_) _(chamber)(Ŝ_(primary,max))  Eq. (12)and the displacement volume of the secondary gas chamber 140 atŜ_(primary,max) may be determined as follows:ΔV _(secondary) _(_) _(chamber) =V _(secondary) _(_) _(chamber) _(_)_(nom) −V _(secondary) _(_) _(chamber)(Ŝ _(primary,max))  Eq. (13)

With combined reference to FIG. 4B and FIG. 8, health monitor 230 mayuse a dynamic airspring model 810, along with a numerical optimizationmethod 820 to estimate the volume of the oil in the secondary gaschamber 140. In this regard, FIG. 8 illustrates an algorithm 800 forestimating the volume of oil leaked into secondary gas chamber 140 (alsoreferred to herein as a secondary chamber oil leakage volume). Dynamicairspring model 810 may generate an accurate estimation of transient gaspressure for compression of a separated gas chamber in which mixing ofair and oil does not happen.

Health monitoring 230 may use the secondary gas chamber inflationpressure (i.e., {circumflex over (P)}_(secondary)(0)), the secondary gaschamber temperature (i.e., {circumflex over (T)}_(secondary)(0)), thedisplacement volume of the secondary gas chamber 140 at Ŝ_(primary,max),computed by equation 13, the nominal volume of the secondary gas chamberwith its piston bottomed out (i.e., V_(secondary) _(_) _(chamber) _(_)_(nom)) and an initial estimate for the volume of the oil leakage intothe secondary gas chamber 140 (i.e., V_(secondary) _(_) _(chamber) _(_)_(leakage)), for example 0 cubic inches, and may compute the secondarychamber pressure at Ŝ_(primary,max).

A gradient free, single-variable, numerical optimization technique, suchas Bisection or Line Search methods may be used to adjust the initialestimate for the oil leakage so that the difference between the measuredsecondary gas chamber pressure at Ŝ_(primary,max) (i.e., {circumflexover (P)}_(secondary)(Ŝ_(primary,max))) (see FIG. 5) and the estimatedsecondary gas chamber pressure at Ŝ_(primary,max) (i.e., P_(secondary@Ŝ) _(primary,max) ) is minimized.

The optimization loop may continue until the absolute difference betweenthe estimated pressure and the measured pressure at Ŝ_(primary,max) isless than or equal to a pre-determined threshold as follows:|{circumflex over (P)} _(secondary@Ŝ) _(primary,max) −P _(secondary@Ŝ)_(primary,max) ≤Threshold 3  Eq. (14)

When equation 14 is satisfied, the last estimate for the oil leakage(i.e., V_(secondary) _(_) _(chamber) _(_) _(leakage)) may be recorded.In various embodiments, if V_(secondary) _(_) _(chamber) _(_)_(leakage)<0, which may possibly be caused by measurement errors, thenoil leakage into the secondary gas chamber will be considered zero.

With the volume of oil leakage into the secondary gas chamber 140 beingdetermined, the gas volume in the secondary gas chamber 140 in the fullyextended position may be determined as follows:V _(secondary) _(_) _(chamber)(0)=V _(secondary) _(_) _(chamber) _(_)_(nom) −V _(secondary) _(_) _(chamber) _(_) _(leakage)  Eq. (15)

The number of moles of gas in the secondary gas chamber 140 of the shockstrut 100 may then be computed using the following equation:

$\begin{matrix}{n_{{secondary}\_{chamber}} = \frac{{{\hat{P}}_{secondary}(0)} \times {V_{{secondary}\_{chamber}}(0)}}{R \times {{\hat{T}}_{secondary}(0)} \times {Z\left( {{{\hat{P}}_{secondary}(0)},{{\hat{T}}_{secondary}(0)}} \right)}}} & {{Eq}.\mspace{14mu}(16)}\end{matrix}$where R is the ideal gas constant and Z({circumflex over(P)}_(secondary)(0), {circumflex over (T)}_(secondary)(0)) is thenitrogen compressibility factor (or the compressibility factor for thetype of gas used in shock strut 100). The computed number of moles ofgas in the secondary gas chamber 140 may be then logged in the datalogger 240. Although {circumflex over (P)}_(secondary)(0) and{circumflex over (T)}_(secondary)(0) are described herein as beingmeasured during a landing event, it is contemplated herein that they mayalso be recorded after a takeoff event. In this regard, {circumflex over(P)}_(secondary)(0) and {circumflex over (T)}_(secondary)(0) aresecondary chamber gas pressure and temperature, respectively, when theshock strut 100 is in the fully extended position (or within 25% of thefully extended position) recorded either during a landing event or atakeoff event. It is noteworthy that instead of secondary chambertemperature, {circumflex over (T)}_(secondary)(0), the primary chambertemperature, {circumflex over (T)}_(primary)(0), may be used tocalculate the number of moles of gas in the secondary gas chamber 140.Shock Strut Servicing State Determination:

In the next step, the oil volume estimated by Eq. 4 may be adjusted to areference temperature, such as 20° C. (68° F.) using the followingthermal model:

$\begin{matrix}{V_{{oil}@T_{ref}} = {V_{oil} \times \left( {1 + {{dT} \times \alpha \times {{sign}\left( {T_{ref} - T_{oil}} \right)}}} \right)^{\frac{{T_{ref} - T_{oil}}}{dT}}}} & {{Eq}.\mspace{14mu}(17)}\end{matrix}$where α is the oil thermal expansion coefficient, dT is a numericalintegration step, and T_(oil) is the oil temperature. T_(oil) may bederived from {circumflex over (T)}_(primary) or {circumflex over(T)}_(secondary). The oil volume computed above may be then logged inthe data logger 240.

The deviation of the oil volume 151 from the nominal oil volume may becomputed as follows:

$\begin{matrix}{{\%\;{oil}} = \frac{V_{{oil}@T_{ref}} - V_{{oil}\_{nom}}}{V_{{oil}\_{nom}}}} & {{Eq}.\mspace{14mu}(18)}\end{matrix}$where V_(oil) _(_) _(nom) is the nominal oil volume which is known tothe algorithm. In various embodiments, the nominal oil volume may be adesired volume of the oil volume 151 of shock strut 100. The deviationof the oil volume 151 from the nominal oil volume may be logged in datalogger 240.

In the next step, the estimated oil volume at the reference temperature(output of Eq. 17) may be compared with a plurality of thresholds, suchas four thresholds as used in the example herein, to determine if theestimated oil volume is acceptable and a proper servicing message may beissued as follows:

-   -   if V_(oil@T) _(ref) >V_(threshold,1)→oil is extremely        overserviced, re-servicing is required    -   if V_(threshold,1)−V_(oil@Tref)>V_(threshold,2)→oil is        overserviced, re-servicing is recommanded    -   if V_(threshold,2)−V_(oil@Tref)>V_(threshold,3)→oil volume is        ok—no action is required    -   if V_(threshold,3)−V_(oil@Tref)>V_(threshold,4)→oil is        underserviced—prepare for servicing    -   if V_(threshold,4)≥V_(oil@Tref)→oil is extremely        underserviced—servicing is required.        The issued servicing message may be logged in the data logger        240.

In the next step, the number of moles of gas in the primary gas chamber130 estimated by Eq. (6) may be compared with the nominal number ofmoles of gas calculated with the following equation:

$\begin{matrix}{n_{{{primary}\_{chamber}}{\_{nominal}}} = \frac{n_{{primary}\_{nom}} \times n_{{{primary}\_{chamber}}{\_{nom}}}}{R \times T_{ref} \times {Z\left( {P_{{primary}\_{nom}},T_{ref}} \right)}}} & {{Eq}.\mspace{14mu}(19)}\end{matrix}$

The deviation of the primary chamber gas level from the nominal valuemay be computed as follows:

$\begin{matrix}{{\%\;{primary}_{gas}} = \frac{n_{{primary}\_{chamber}} - n_{{{primary}\_{chamber}}{\_{nominal}}}}{n_{{{primary}\_{chamber}}{\_{nominal}}}}} & {{Eq}.\mspace{14mu}(20)}\end{matrix}$The deviation of the primary gas level from the nominal level may belogged in data logger 240.

The estimated number of moles of gas in the primary gas chamber 130 maybe compared with a plurality of thresholds, such as four thresholds asused in the example herein, and a proper servicing message is issued asfollows:

-   -   if n_(primary) _(_) _(chamber)>n_(primary,threshold,1)→primary        chamber is extremely overserviced, re-servicing is required    -   if n_(primary,threshold,1)≥n_(primary) _(_)        _(chamber)>n_(primary,threshold,2)→primary chamber is        overserviced, re-servicing is recommended    -   if n_(primary,threshold,2)≥n_(primary) _(_)        _(chamber)>n_(primary,threshold,3)→primary chamber gas level is        ok—no action is required    -   if n_(primary,threshold,3)≥n_(primary) _(_)        _(chamber)>n_(primary,threshold,4)→primary chamber is        underserviced—prepare for servicing

if n_(primary,threshold,4)≥n_(primary-chamber)→primary chamber isextremely underserviced—servicing is required.

The issued servicing message may be logged in data logger 240.

In the next step, the number of moles of gas in the secondary gaschamber 140 estimated by Eq. (14) may be compared with the nominalnumber of moles of gas calculated with the following equation:

$\begin{matrix}{n_{{{secondary}\_{chamber}}{\_{nominal}}} = \frac{n_{{primary}\_{nom}} \times V_{{{primary}\_{chamber}}{\_{nom}}}}{R \times T_{ref} \times {Z\left( {P_{{secondary}\_{nom}},T_{ref}} \right)}}} & {{Eq}.\mspace{14mu}(21)}\end{matrix}$

The deviation of the secondary chamber gas level from the nominal valuemay be computed as follows:

$\begin{matrix}{{\%\;{{secondary}{\_ gas}}} = \frac{n_{{secondary}\_{chamber}} - n_{{{secondary}\_{chamber}}{\_{nominal}}}}{n_{{{secondary}\_{chamber}}{\_{nominal}}}}} & {{Eq}.\mspace{14mu}(22)}\end{matrix}$

The deviation of the secondary gas level from the nominal level may belogged in data logger 240.

The estimated number of moles of gas in the secondary gas chamber 140may be compared with four thresholds (or any other number of thresholds)and a proper servicing message may be issued as follows:

-   -   if n_(secondary) _(_)        _(chamber)>n_(secondary,threshold,1)→secondary chamber is        extremely overserviced, re-servicing is required    -   if n_(secondary,threshold,1)≥n_(secondary) _(_)        _(chamber)>n_(secondary,threshold,2)→secondary chamber is        overserviced, re-servicing is recommended    -   if n_(secondary,threshold,2)≥n_(secondary) _(_)        _(chamber)>n_(secondary,threshold,3)→secondary chamber gas level        is ok—no action is required    -   if n_(secondary,threshold,3)≥n_(secondary) _(_)        _(chamber)>n_(secondary,threshold,4)→secondary chamber is        underserviced—prepare for servicing    -   if n_(secondary,threshold,4)≥n_(secondary) _(_)        _(chamber)→secondary chamber is extremely        underserviced—servicing is required.        The issued servicing message may be logged in data logger 240.

The volume of oil leakage into the primary gas chamber 130 (calculatedby Eq. 11) may be compared with a plurality of thresholds and a properservicing message may be issued as follows:

-   -   if V_(primary) _(_) _(chamber) _(_) _(leakage) _(_) _(threshold)        _(_) ₁>V_(primary) _(_) _(chamber) _(_) _(leakage)→no leakage,        no action is required    -   if V_(primary) _(_) _(chamber) _(_) _(leakage) _(_) _(threshold)        _(_) ₂>V_(primary) _(_) _(chamber) _(_) _(leakage)→V_(primary)        _(_) _(chamber) _(_) _(leakage) _(_) _(threshold) _(_) ₁→some        leakage into the primary chamber, prepare for inspection    -   if V_(primary) _(_) _(chamber) _(_) _(leakage)≥V_(primary) _(_)        _(chamber) _(_) _(leakage) _(_) _(threshold) _(_) ₂→excessive        leakage into the primary chamber, inspection is required        The issued servicing message may be logged in data logger 240.

The volume of oil leakage into the secondary gas chamber 140 recorded bymonitoring system 200 may be compared with a plurality of thresholds anda proper servicing message may be issued as follows:

-   -   if V_(secondary) _(_) _(chamber) _(_) _(leakage) _(_)        _(threshold) _(_) ₁>V_(secondary) _(_) _(chamber) _(_)        _(leakage)→no leakage, no action is required    -   if V_(secondary) _(_) _(chamber) _(_) _(leakage) _(_)        _(threshold) _(_) ₂>V_(secondary) _(_) _(chamber) _(_)        _(leakage)≥V_(secondary) _(_) _(chamber) _(_) _(leakage) _(_)        _(threshold) _(_) ₁→some leakage into the secondary chamber,        prepare for inspection    -   if V_(secondary) _(_) _(chamber) _(_) _(leakage)≥V_(secondary)        _(_) _(chamber) _(_) _(leakage) _(_) _(threshold) _(_)        ₂→excessive leakage into the secondary chamber, inspection is        required        The issued servicing message may be logged in data logger 240.

With reference to FIG. 9, a method 900 for monitoring a shock strut isprovided, in accordance with various embodiments. Method 900 includesreceiving a plurality of sensor readings (step 910). Method 900 includesdetermining a shock strut stroke at which a secondary chamber isactivated (step 920). Method 900 includes calculating a volume of oil inan oil chamber (step 930). Method 900 includes calculating a volume ofgas in a primary chamber (step 940). Method 900 includes calculating anumber of moles of gas in the primary chamber (step 950). Method 900includes calculating a volume of oil leaked into the primary chamber(step 960). Method 900 includes calculating a volume of gas in asecondary chamber (step 970). Method 900 includes calculating a volumeof oil leaked into the secondary chamber (step 980). Method 900 includescalculating a number of moles of gas in the secondary chamber (step990).

With combined reference to FIG. 4A, FIG. 4B, and FIG. 9, step 910 mayinclude receiving, by controller 201, primary chamber gas pressure 250,primary chamber gas temperature 252, secondary chamber gas pressure 254,secondary chamber gas temperature 256, and/or shock strut stroke 258.Step 920 may include determining, by controller 201, S_(activation).S_(activation) may be determined using equation 2, as described herein.Step 930 may include determining, by controller 201, oil volume 151using algorithm 600 (see FIG. 6), as described herein. Step 940 mayinclude calculating, by controller 201, volume of gas 131 in primary gaschamber 130, as described herein. Step 950 may include calculating, bycontroller 201, a number of moles of gas in the primary gas chamber 130using Eq. 6, as described herein. Step 960 may include calculating, bycontroller 201, a volume of oil leaked into the primary gas chamber 130(i.e., V_(primary) _(_) _(chamber) _(_) _(leakage)) using equation 11,as described herein. Step 970 may include calculating, by controller201, a volume of gas 141 in secondary gas chamber 140 using equation 12,as described herein. Step 980 may include calculating, by controller201, a volume of oil leaked into the secondary chamber (i.e.,V_(secondary) _(_) _(chamber) _(_) _(leakage)) using algorithm 800, asdescribed herein. Step 990 may include calculating, by controller 201, anumber of moles of gas in the secondary gas chamber 140, using equation21, as described herein.

Benefits, other advantages, and solutions to problems have beendescribed herein with regard to specific embodiments. Furthermore, theconnecting lines shown in the various figures contained herein areintended to represent exemplary functional relationships and/or physicalcouplings between the various elements. It should be noted that manyalternative or additional functional relationships or physicalconnections may be present in a practical system. However, the benefits,advantages, solutions to problems, and any elements that may cause anybenefit, advantage, or solution to occur or become more pronounced arenot to be construed as critical, required, or essential features orelements of the disclosure.

The scope of the disclosure is accordingly to be limited by nothingother than the appended claims, in which reference to an element in thesingular is not intended to mean “one and only one” unless explicitly sostated, but rather “one or more.” It is to be understood that unlessspecifically stated otherwise, references to “a,” “an,” and/or “the” mayinclude one or more than one and that reference to an item in thesingular may also include the item in the plural. All ranges and ratiolimits disclosed herein may be combined.

Moreover, where a phrase similar to “at least one of A, B, and C” isused in the claims, it is intended that the phrase be interpreted tomean that A alone may be present in an embodiment, B alone may bepresent in an embodiment, C alone may be present in an embodiment, orthat any combination of the elements A, B and C may be present in asingle embodiment; for example, A and B, A and C, B and C, or A and Band C.

The steps recited in any of the method or process descriptions may beexecuted in any order and are not necessarily limited to the orderpresented. Furthermore, any reference to singular includes pluralembodiments, and any reference to more than one component or step mayinclude a singular embodiment or step. Elements and steps in the figuresare illustrated for simplicity and clarity and have not necessarily beenrendered according to any particular sequence. For example, steps thatmay be performed concurrently or in different order are illustrated inthe figures to help to improve understanding of embodiments of thepresent disclosure.

Any reference to attached, fixed, connected or the like may includepermanent, removable, temporary, partial, full and/or any other possibleattachment option. Additionally, any reference to without contact (orsimilar phrases) may also include reduced contact or minimal contact.Surface shading lines may be used throughout the figures to denotedifferent parts or areas but not necessarily to denote the same ordifferent materials. In some cases, reference coordinates may bespecific to each figure.

Systems, methods and apparatus are provided herein. In the detaileddescription herein, references to “one embodiment”, “an embodiment”,“various embodiments”, etc., indicate that the embodiment described mayinclude a particular feature, structure, or characteristic, but everyembodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed. After reading the description, it will be apparent to oneskilled in the relevant art(s) how to implement the disclosure inalternative embodiments.

Furthermore, no element, component, or method step in the presentdisclosure is intended to be dedicated to the public regardless ofwhether the element, component, or method step is explicitly recited inthe claims. No claim element is intended to invoke 35 U.S.C. 112(f)unless the element is expressly recited using the phrase “means for.” Asused herein, the terms “comprises”, “comprising”, or any other variationthereof, are intended to cover a non-exclusive inclusion, such that aprocess, method, article, or apparatus that comprises a list of elementsdoes not include only those elements but may include other elements notexpressly listed or inherent to such process, method, article, orapparatus.

What is claimed is:
 1. A monitoring system for a dual-stage, separatedgas/fluid shock strut, comprising: a controller; and a tangible,non-transitory memory configured to communicate with the controller, thetangible, non-transitory memory having instructions stored thereon that,in response to execution by the controller, cause the controller toperform operations comprising: receiving, by the controller, a primarychamber temperature sensor reading from a first sensor coupled to aprimary chamber of the dual-stage, separated gas/fluid shock strut;receiving, by the controller, a primary chamber pressure sensor readingfrom at least one of the first sensor and a second sensor coupled to theprimary chamber of the dual-stage, separated gas/fluid shock strut;receiving, by the controller, a secondary chamber pressure sensorreading from a third sensor coupled to a secondary chamber of thedual-stage, separated gas/fluid shock strut; receiving, by thecontroller, a shock strut stroke sensor reading from a stroke sensor;determining, by the controller, a shock strut stroke at which thesecondary chamber of the dual-stage, separated gas/fluid shock strut isactivated, wherein the shock strut stroke is determined in response to astrut piston moving with respect to a strut cylinder; calculating, bythe controller, a volume of oil in an oil chamber of the shock strutbased upon the shock strut stroke at which the secondary chamber of thedual-stage, separated gas/fluid shock strut is activated; calculating,by the controller, a primary chamber gas volume in the primary chamberof the dual-stage, separated gas/fluid shock strut; calculating, by thecontroller, a primary chamber oil leakage volume of oil leaked into theprimary chamber of the dual-stage, separated gas/fluid shock strut;comparing, by the controller, the primary chamber oil leakage volume ofoil leaked into the primary chamber with a plurality of primary chamberoil leakage threshold values; and issuing, by the controller, aservicing message, in response to the comparing.
 2. The monitoringsystem of claim 1, wherein the instructions cause the controller toperform further operations comprising: calculating, by the controller, anumber of moles of gas in the primary chamber of the dual-stage,separated gas/fluid shock strut, based upon at least the primary chambergas volume.
 3. The monitoring system of claim 2, wherein theinstructions cause the controller to perform further operationscomprising: comparing, by the controller, the number of moles of gas inthe primary chamber with a plurality of primary chamber thresholdvalues; and issuing, by the controller, a primary chamber servicingmessage, in response to the comparing.
 4. The monitoring system of claim1, wherein the instructions cause the controller to perform furtheroperations comprising: receiving, by the controller, a secondary chambertemperature sensor reading; calculating, by the controller, a secondarychamber gas volume in the secondary chamber; and calculating, by thecontroller, a volume of oil leaked into the secondary chamber of thedual-stage, separated gas/fluid shock strut, based upon a nominalvolume, based upon at least one of the secondary chamber pressure sensorreading, the secondary chamber temperature sensor reading, a displacedvolume of the secondary chamber, and a total volume of the secondarychamber.
 5. The monitoring system of claim 4, wherein the instructionscause the controller to perform further operations comprising:calculating, by the controller, a number of moles of gas in thesecondary chamber of the dual-stage, separated gas/fluid shock strut,based upon at least the secondary chamber gas volume.
 6. The monitoringsystem of claim 5, wherein the instructions cause the controller toperform further operations comprising: comparing, by the controller, thenumber of moles of gas in the secondary chamber with a plurality ofsecondary chamber threshold values; and issuing, by the controller, asecondary chamber servicing message, in response to the comparing. 7.The monitoring system of claim 4, wherein the instructions cause thecontroller to perform further operations comprising: calculating, by thecontroller, the displaced volume of the secondary chamber of thedual-stage, separated gas/fluid shock strut.
 8. The monitoring system ofclaim 4, wherein the instructions cause the controller to performfurther operations comprising: comparing, by the controller, the volumeof oil in the oil chamber with a plurality of threshold values; andissuing, by the controller, an oil volume servicing message, in responseto the comparing.
 9. The monitoring system of claim 4, wherein theinstructions cause the controller to perform further operationscomprising: comparing, by the controller, the volume of oil leaked intothe secondary chamber with a plurality of secondary chamber oil leakagethreshold values; and issuing, by the controller, a secondary chamberservicing message, in response to the comparing.
 10. The monitoringsystem of claim 1, wherein the instructions cause the controller toperform further operations comprising adjusting the volume of oil in theoil chamber to a reference temperature.
 11. The monitoring system ofclaim 1, wherein the instructions cause the controller to performfurther operations comprising calculating, by the controller, adeviation of the volume of oil in the primary chamber from a nominal oilvolume for the oil chamber.